1. Technical Field
The present invention relates to a process for the production of a layer structure of a nitride semiconductor component on a silicon surface. The invention also relates to a process for the production of a nitride semiconductor component. Finally, the invention relates to a nitride semiconductor component, in particular a thin film light-emitting diode (LED) based on a nitride semiconductor, as well as a nitride semiconductor product.
2. Discussion of Related Art
Nitride semiconductors are semiconductor compounds that contain one or more elements of the third main group as well as one or more elements of the fifth main group of the Periodic System. Such semiconductors include for example the semiconductors GaN, InGaN, InGaAsN, AlGaN, etc. Other common designations for nitride semiconductors within the context of the present invention are Group III nitrides and II-V nitrides. These designations are used interchangeably in the present application and have the same meaning.
Nitride semiconductors are used in particular in light-emitting structures that emit in the visible and ultraviolet regions of the spectrum. Apart from this electronic nitride semiconductor components are also known, such as for example high electron mobility (HEM) transistors, which are suitable in particular for high-frequency applications such as for example in radio transmission technology. Nitride semiconductor components are also used in the form of so-called “power devices” in high-power electronics.
On account of the very small size and poor quality of available substrates of nitride semiconductors there is at the present time little commercial interest in an inexpensive homoepitaxy of the layer structure of a nitride semiconductor component. Available nitride semiconductor components, such as for example blue or green LEDs, therefore contain layer structures that are deposited on sapphire (Al2O3) substrates or silicon carbide (SiC) substrates. These substrate materials have various disadvantages. On the one hand they are expensive. On the other hand commonly available substrates of these materials are comparatively small, so that the production costs per component are in addition increased on account of the relatively small yield for a given substrate surface. Added to this is the significant hardness of these materials, which is above 9 on Moh's scale and permits a mechanical treatment only with expensive diamond saws and grinding materials.
Silicon substrates are therefore increasingly used for a large-area growth, and as is known such substrates with a large diameter can be obtained inexpensively.
Typical growth temperatures for the layer structures of nitride semiconductors in the commercially normally employed gaseous phase epitaxy are above 1000° C. Different thermal coefficients of expansion of silicon and nitride semiconductor materials then lead during cooling of the deposited nitride semiconductor layer structures after the growth stage to a high tensile stress of the nitride semiconductor layers of about 0.7 GPa/μm and, starting with layer thicknesses of less than 1 μm, to crack formation.
In order to avoid crack formation during the growth of GaN layers on silicon, thin intermediate layers grown at unusually low temperatures (below 1000° C.), so-called low-temperature AlN or AlGaN intermediate layers, are used. The effect of these layers is based on a partial compensation of the tensile stress due to growth of a GaN layer with compressive stress on the AlN or AlGaN intermediate layer. During cooling after the layer deposition this compressive stress of the GaN layer counteracts the tensile stress produced by the different thermal coefficients of expansion, and leads as a result to a reduced tensile stress.
A disadvantage of this technique is a high dislocation concentration in the GaN layer growing on the intermediate layer. In DE 101 51 092 A1, which is hereby incorporated by reference into the disclosure of the present application, the additional insertion of silicon nitride intermediate layers into the growing GaN layer is therefore proposed in order to reduce the dislocation density. A not necessarily completely closed SixNy intermediate layer serves as a mask for subsequent growth of GaN. The thickness of the intermediate layer is according to DE 101 51 092 A1 chosen so that only a few growth islands spaced apart by 100 nm up to a few μm are formed on it, which during the further course of the growth, with increasing distance of the growth surface from the SiN intermediate layer have grown together starting from a so-called coalescence thickness and form a closed layer surface. Of course, only a SiN intermediate layer in the GaN epitaxy on silicon produces a pronounced island growth and thus a significant coalescence thickness, which grows with increasing SiN thickness. By suitable measures to accelerate the coalescence of these growth islands the aforementioned critical crack thickness can however already be prevented from being reached before the growth islands coalesce.
From the document by A. Dadgar et al., “Reduction of Stress at the Initial Stages of GaN Growth on Si(111)”, Applied Physics Letters, Vol. 82, 2003, No. 1, pp. 28-30, (hereinafter briefly referred to as “Dadgar et al.”), which is hereby incorporated by reference into the disclosure of the present application, it is furthermore known to produce GaN layers after deposition of a silicon-doped AlN nucleation layer and an SiN masking layer of at most about 1.5 monolayers nominal thickness. The tensile stress in the growing GaN layer can be reduced compared to a growth without such a SiN masking layer.
This effect exhibits, as a function of the SiN masking layer thickness, saturation phenomena starting from a certain thickness, and a complete stress compensation cannot be expected. The fact is, on the one hand the SiN masking layer can, as its thickness increases, interfere with or even prevent the structural coupling between the AlN nucleation layer and the subsequently (i.e. after the SiN masking layer) growing GaN layer. The result is that a desired compressive effect on the AlN nucleation layer may then no longer occur and an undesirably high tensile stress remains in the final nitride semiconductor layer. Secondly, a thick SiN layer increases the coalescence thickness to values that, with the known methods, can no longer be kept under the critical layer thickness for crack formation.
As a result, the insertion also of a SiN masking layer can accordingly no longer eliminate the tensile stress in the nitride semiconductor layer structure.
Non-homogeneous tensile stress has further disadvantages. Apart from the already-mentioned high dislocation concentration, it also causes a curvature of the growing layer structure and of the underlying substrate. This problem also affects thin layer components such as thin layer LEDs, in which the silicon substrate is removed during the course of manufacture. The processing of curved nitride semiconductor layer structure already cause problems and therefore increases the complexity and costs of component manufacture. A curved nitride semiconductor layer structure that is then typically bonded to a carrier easily becomes detached from the carrier, and corresponding components have an undesirably short useful life.
From the document by C. Mo et al., “Growth and characterization of InGaN blue LED structure on Si(111) by MOCVD”, Journal of Crystal Growth, 285 (2005), 312-317 (hereinafter briefly referred to as “Mo et al.”), which is hereby incorporated by reference into the disclosure of the present application, it is known to reduce the tensile stress by growing a GaN buffer layer on the AlN nucleation layer. In this case a very low ratio of the gas flow densities of gallium precursor to nitrogen precursor are adjusted for the growth of the GaN buffer layer in the high-temperature gaseous phase epitaxy. This promotes the island growth of the subsequent GaN layer. The disadvantage here however is that tensile stresses still remain in the nitride semiconductor layer structure. In addition this known LED has an undesirably high electrical resistance.
The technical problem on which the present invention is based is accordingly to provide a process for the production of a layer structure of a nitride semiconductor component on a silicon surface and a nitride semiconductor component itself, in which the tensile stress in the finished layer structure is further reduced compared to known solutions to this problem.
A further technical problem on which the present invention is based is to provide a process for the production of a layer structure of a nitride semiconductor component on a silicon surface and a nitride semiconductor component itself that reduces the curvature of the nitride semiconductor layer structure compared with known solutions to this problem.